Carefully sized rectangular cuts (20 mm wide) increase the adhesion of a polydimethylsiloxane strip by 10 times along its length.

Credit: Michael D. Bartlett

Carefully sized rectangular cuts (20 mm wide) increase the adhesion of a polydimethylsiloxane strip by 10 times along its length.

Credit: Michael D. Bartlett

Borrowing a page from the Japanese paper-cutting art of kirigami, researchers have made tape that is 10 times as sticky as uncut tape but is also easy to pull free and then reuse (ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.7b18594). The reversible adhesive could be used to make wall-climbing robots, wearable tattoolike sensors, and bandages that come off without making you wince.

Kirigami has recently gained the attention of engineers and materials scientists as a tool for designing unusual devices and materials. The technique can be used to create airy, interconnected geometrical structures that are mechanically strong. Researchers have used the paper-cutting technique to make stretchable batteries and conductors; solar panels with movable, sun-tracking solar cells; and complex three-dimensional structures that pop up from flat sheets.

Michael D. Bartlett and his colleagues at Iowa State University of Science & Technology wanted to apply kirigami concepts to control adhesion. They found that putting cleverly designed cuts in a clingy film make it stick strongly but release easily when pulled in a specific direction. “It’s counterintuitive,” Bartlett says. “You would think cutting the tape would make it less adhesive, but well-designed cuts let you enhance and precisely control adhesion. The kirigami structures influence how much force you need to apply to remove the material.”

Others have made strong, reversibly adhesive tape that uses van der Waals forces to stick to surfaces—inspired by the microstructures on the bristles that cover gecko toe pads. But making those 3-D structures requires complex procedures and equipment. The new approach also relies on van der Waal’s forces for its stickiness but uses simple sheets of plastic film and fast laser cutting.

Bartlett and his colleagues sandwiched a 0.75-mm-thick polyethylene film between flexible polydimethylsiloxane sheets. Then they laser-cut it with a simple pattern consisting of two columns of periodic rectangles running the length of the tape like windows.

By carefully experimenting with the spacing and dimensions of the rectangular cuts and the areas of tape around them, the researchers fine-tuned the sheet’s reversible stickiness. They got the strongest adhesion when the tape had thick divisions between the tops and bottoms of the windows and thinner divisions along the edges of the tape and between the two rectangular columns.

Bartlett explains that as you bend and peel off the strip, the tape requires greater force to remove whenever the bent part moves from an open region to the stiffer tape, compared with the force needed to peel off an uncut, solid region. Along the tape’s length, these recurring transitions boost the stickiness. That’s not the case along its width; in that direction, the tape is removed more easily than one that’s uncut. This provides a new way to both enhance and reduce stickiness in a single adhesive. By adding a tacky glue layer like the ones found on Scotch tape or medical tape, the kirigami-inspired tape would in theory be stronger than those products, Bartlett says.

As a demonstration, they mounted their tape on a volunteer’s arm. When tugged along its length, it needs 10 times as much force to detach as a plain, uncut tape. But when peeled across its width, it lifts off easily. The team now wants to study these effects at smaller dimensions, with different shapes, and with shapes that aren’t linearly arranged to see how that would change adhesion.

The use of kirigami to make tunable adhesives is novel, says Douglas P. Holmes, professor of mechanical engineering at Boston University. “The advantage with this approach is that instead of tuning material properties, engineers can simply tune the geometry of existing materials,” he says. Additional work will be needed to understand the stresses around the edges of the kirigami cuts, he adds.